Parasequence-Scale Organic Facies Variations and Its Significance in Nonmarine Basin

YANG Ming hui; XIA Wen chen; ZHANG Bing shan; ZHANG Ning; PENG Xiao qun

The Jurassic of the northern Qaidam basin is widely recognized as the nonmar ine sediments, and interpreted as alluvial-delta and lacustrine depositional system. The depositional sequence of the Tianshuigou Fm. within the lower Jura ss ic mainly composed of the alluvial system at the base and lower and the deep lac ustrine system in the upper. Geochemical and optical analyses were undertaken on the minor cyclothems, a progradationally coarsening-and shallowing-upward paraseque nce (cycle) within the major lacustrine deposits in which Type III land-plant derived kerogen is overwhelming. The total organic carbon cont ent of the fine-grainned lithology increases upwards through the parasequence and is positively correlated with silt content, presumably due to the hydrodynam ic equivalence and common alluvial source of phytoclasts and silt particles. The section is marginally mature ( about 0.57% vitrinite reflectance). The distri bution of pyrite and siderite is apparently related to sediment accumulation rate,with frequent appearance of siderite in the lower part of the para sequence. The lacustri ne flooding surface at the base of the parasequence is overlain by black shale, and thought to have been terminated by an abrupt and major rise in relative lac ustrine level,which may be perhaps the result of tectonically driven changes in subsidence.

1 Northwest University, Xi'an 710069;

2 China University of Geosciences, Wuhan 430074;

3 Qinghai Petroleum Administration Bureau, Dunhuang Gansu 743000

Received Date: 1998-11-26

Rev Recd Date: 1999-03-26

Publish Date: 2000-06-10

Key words:

Abstract: The Jurassic of the northern Qaidam basin is widely recognized as the nonmar ine sediments, and interpreted as alluvial-delta and lacustrine depositional system. The depositional sequence of the Tianshuigou Fm. within the lower Jura ss ic mainly composed of the alluvial system at the base and lower and the deep lac ustrine system in the upper. Geochemical and optical analyses were undertaken on the minor cyclothems, a progradationally coarsening-and shallowing-upward paraseque nce (cycle) within the major lacustrine deposits in which Type III land-plant derived kerogen is overwhelming. The total organic carbon cont ent of the fine-grainned lithology increases upwards through the parasequence and is positively correlated with silt content, presumably due to the hydrodynam ic equivalence and common alluvial source of phytoclasts and silt particles. The section is marginally mature ( about 0.57% vitrinite reflectance). The distri bution of pyrite and siderite is apparently related to sediment accumulation rate,with frequent appearance of siderite in the lower part of the para sequence. The lacustri ne flooding surface at the base of the parasequence is overlain by black shale, and thought to have been terminated by an abrupt and major rise in relative lac ustrine level,which may be perhaps the result of tectonically driven changes in subsidence.

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Reference (13)

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Parasequence-Scale Organic Facies Variations and Its Significance in Nonmarine Basin

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